Method of coagulating latices of synthetic rubberlike materials



Allg. 19, 1947. R M, THQMAS 5TM,` 2,426,127

IETHOD 0F COAGULATING LATICES OF SYNTHETIC RUYBBBRLIKE AT ERIAXJSv Filedct. 29, 1941 V Flin INLET COAG ULA TIO/V YSSLL BLOW uns:

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Patented Aug. 19, 1947 METHOD OF COAGULATING LATICES F SYNTHETIC RUBBERLIKE MATERIALS l Robert M. Thomas, Union, and Donald C. Field, Linden, N. J., assignors, by mesne assignments, to Jasco, Incorporated, a corporation of Louisiana Application October 29, 1941, Serial No. 416,978

13 Claims. (Cl. 260-93) l 'I'his invention relates to the preparation of synthetic rubber-like materials; and has the object of providing a method or coagulating synthetic rubber-like polymerization products more 'emcienctly and expeditiously.

The method of this invention is of technical and practical importance in the manufacture of synthetic rubber-like materials particularly of the Buna type vby emulsion polymerization processes. It is of the particular importance as applied to the emulsion polymerization product of butadiene and acrylonitrile; butadiene and styrene; and other similar mixtures that contain the polymerizate highly dispersed in a liquid reaction medium requiring coagulation of th'e dispersed polymers for their recovery.

In a narrower sense, the high molecular weight materials having rubber-like elastic properties, which are of primary interest, are products whos'e basic copolymerized components comprise dienes (dioleiins closely related to butadiene), suclras isoprene, dimethyl butadiene, piperylene, or other polyolens capable of interpolymerization with other polymerizable substances. Such products can be irreversibly converted from a plastic to elastic form by vulcanizing. 'I'hey are identinedfin general, by their higher stability as compared to natural rubber.

In accordance with the present invention, coagulation of a highly dispersed polymerizate, as in the form of a latex or emulsion, is effected by y subdividlng the emulsion'or mixture of a liquid polymerization,` reaction medium containing a dispersedpolymerizate to a controlled suitable extent and contacting the material thus subdivided Wth an immiscible liquid at temperatures suillciently low to induce coagulation of the polymer and freezing of the liquid emulsion medium. Y

Known methods of coagulating emulsiiied synthetic rubber-like polymers, involve a number of complications and are dimcult to control for securing coagulates of optimum small particle size. The coagulates obtained tend to be gummy and lumpy, making wash'ing diilicult and even requiring special equipment, such as washingv mills. For example, in the much used coagulation of synthetic polymers by the addition of cold brine, the salt must be of high purity and the adjustment of the pH value is highly critical.

Before the usual coagulation step, preliminary 4steps of stripping and removing unconverted reactants from the emulsion have been necessary. After the coagulation is accomplished with cold brine, much washing isA required before the coagulate is satisfactorily free of brine andl brine soaps, the brine having to be removed iirst and then the soaps. Furthermore, when certain agents known as'modiilers are used in the polymerization reaction, and the polymer is precipitated by the usual methods, such as by cold brine, the coagulate is excessively sticky, thus rendering subsequent washing. filtering, and drying operations diillcult or virtually impossible. With acids as the coagulants in place of salt, free acids liberated from the emulsier are frequently very objectionable.' With efforts to coagulate simply by freezing with no coagulating agent, a long time is required to obtain complete coagulation. Hence, the coagulation step is of a critical nature, because if the rubber-like product precipitated has an undesirable form, it is hard- It has been found that the coagulation method A of the present invention satisfactorily avoids diillculties and complications of the previous methods and is superior thereto in a number of respects. Other advantages will be further explained in the following description.

In conducting the present method, the liquid mixture or latex containing the dispersed polymerlzate is subdivided into particles of definitely small size in a convenient manner, as, for example, by means of a spray nozzle into or onto a cold liquid coagulating agent of proper character.

The liquid used as the coagulating agent is a material which is inert and immiscible with the liquid medium carrying the dispersed polymer as well as inert to the polymer under the coagulating conditions. It is contacted with the spray at a temperature sulciently low to induce freezing of the liquidmedium with attendant' coagulation of the polymer.

The emulsion polymerization products are obtained in the form of a. latex-like emulsion having as the dispersing liquid or continuous phase medium a polar substance, usually water. With such emulsions, the coagulant used in accordance with the present invention is preferably a highly volatile non-polar organic substance substantially immiscible with the water, such as hydrocarbons or non-polar alkyl halides, preferably those which are liquefied at below 0 C. and approximately in the range of -20 C. to C.

For the purpose of illustration, a diagrammatic view is shown of suitable means for carrying out the coagulation step in Figure 1 of the drawing. v Figure 2 is a more detailed and enlarged crossl sectional view of a suitable spray nozzle for atomin the emulsion particles.

i of a slurry.

freezing and coagulating agent (coagulant) is blowy case 9 in which is maintained a suitable pressure supplied by an inert gas, e. g., nitrogen, introduced by line I above the level of the emulsion, line I0 being equipped with regulating valve II and pressure gauge I2. Line I3 supplies the blow case with the emulsion directly or indirectly from the emulsion polymerization reaction zone. It is to be observed that the emulsion forced up under pressure from the bottom of blow case 9 through supply line 2 may be in a stream free 4from the inert gas used to place the emulsion under pressure.

In Figure 2, the enlarged view 0f the spray nozzle 3, previously referred' to as being an end attachment to inlet 2 for atomizing the emulsion within the coagulation vessel I is shown to contain a filter screen I4 of about 40 mesh followed by valve I5, which may be adjusted to obtain a desired rate of iiow of the emulsion stream, and a lower plug member I6 having spiral grooves through which the emulsion is forced into small spiral streams, which leave a smallnozzle orice I1 in an atomized cone shaped spray. Inusing this type of spray device nozzle, it was observed that the emulsion did not tend to coagulate in being sprayed and did not plug up the screen, and it was observed that the atomized droplets of emulsion were distributed in a suitably uniform manner onto the surface of the cold coagulant bath within the coagulation vessel.

With mechanical agitation of the coagulating liquid bath, the particles of emulsion spread onto the surface of the liquid bath were stirred into the liquid, forming therein a suspension vof coagulated polymer having the appearance of small granular particles with interspersed ice crystals formed by the cogealing of the Water of the emulsion. As the spraying of the emulsion is continued for a period into a limited quantity of the coagulating liquid, the suspension becomes increasingly thicker until it reaches a slush-like consistency. However, fresh coagulant liquid may be introduced by inlet 4 at a suflicient rate to replenish any removed by vapon'zation and vapor withdrawal through line 6 or by\removal through the drain pipe l5, so as to keep the slurry suitably flowable for withdrawal continuously or intermittently by drain pipe 5.

The coagulating liquid acting as a self-refrigerant evolves a substantial amount of vapors, particularly when it is kept substantially at its boiling or liquefaction point within the vessel I. the same time, such a coagulating liquid absorbs sensible heat from the atomized emulsion and latent heat developed by the freezing ofthe Water Thus. the coagulating liquid introduced at a suitable rate acts very eiiiciently in maintaining a constant and uniform temperature throughout the coagulation zone and maintains the coagulated and frozen particles. of the emulsion vsatisfactorily suspended in the form The pressure in the coagulation vessel is readily controlled with a pressureregulation valve 'I in the vaporvent line, if desired, for any specific low boiling coagulating liquid employed. The pressure may bemaintained at substantially the equilibrium boiling and liquefaction point of the coagulating liquid. For example, with approximately one atmosphere of pressure maintained in the coagulation vessel using isobutylene as the coagulating liquid, the temperature at which coagulation is made to take place can be maintained quite constantly at about 6 CL; with liquid butadiene as the coagulating liquid under approximately one atmosphere of pressure, the coagulation temperature is maintained constantly at substantially its liquefaction temperature of about 5 C. Thus, it is possible to maintain any desired suitable temperature uniformly and automatically through the use of a speciiic coagulating liquid or mixture oi such liquids under a xed pressure.

Vapors evolved from the coagulating liquid and Withdrawn through vapor line 6 at a suitable rate to maintain the desired pressure in vessel I may be recovered in any convenient manner for further use, as, for example, by recompressing and cooling to liquefy the vapors. The reliquefled vapors may be resupplied through line i to vessel I. Thus, it is advantageous to employ as coagulating liquids, normally gaseous substances that are liquefied at temperatures Wlich are not too far below the freezing point of the emulsion medium, and good economy in refrigeration is obtained.v However, even very low boiling point substances, such as liquefied ethylene, may be used.` On the other hand, higher boiling organic substances which are' immiscible with the emulsion medium may be employed as the coagulatingr liquid with external refrigeration and added internal refrigerant, such as frozen carbon dioxide, but they are also less advantageously used than the preferred substances having liquefaction points below the freezing point of the emulsion medium. A particularly advantageous coagulating liquid for emulsions containing polymers from butadiene -is liquefied butadiene, because its use simplifies the plant operation.

The coagulate suspended-in the coagulation liquid as a flowable slurry is madeto ow by gravity or by pumping to sulbse'quent umts for further processing as, for example, first a washing vessel I8 having a water inlet I9, a vapor' vent-2U and aqueous slurry withdrawal line 2|; which leads to a further processing unit as, for example, to carry out any further washing desired, lterlng, and drying of the washed coagula'te.- Vessel I8 may Aalso be equipped with a stealiffjet inlet 22 for injecting steam into the water and coagulate mixture in order to aid in stripping' therefrom volatile materials, such as any unconverted reactants 5 passed from vessel I to vessel I8, the vessel I8 may be partly filled with warm water, preferably distilled water, supplied by inlet I0, The coagulate slurry is run into the body of warm water, whereupon the volatile coaguiating liquid is rapidly ilashed into vapors which are withdrawn through pipe 20. In meeting with the warm water, the ice contained in the particles of slurry is rapidly melted and the coagulated polymer particles become distributed throughout the warm wash water and become substantially washed, so that `the thus reslurried coagulate can immediately thereafter be subjected to filtration or centrifuging, in order to `separate the water and materials 4dissolved in the water from the coagulated polymer in a satisfactorily washed condition. It has Ibeen noted that with the present method of coagulation, a single Washing step is highly efficient and this may be properly attributed in part at least to the controlled line size of the coagulate particles with lessA of the substances entrapped which have to be separated from the coagulate.

In coagulation procedurea'it has been necessary to subject thel emulsion from the polymerization zone to intermediate operations for removing unconverted reactants, e. g., the diolen and any of the coreacting substances such as acrylonitrile, styrene or other polymerizable compounds having one or more double bonds, before subjecting the` emulsion to coagulation, so as to arrest the interpolymerization reaction, for preventing undesired side reactions or over-polymerization. With the present method, unconverted reactants remaining in the emulsion can be recovered following the coagulation as in the washing step, if

i desired.

There are a number of methods which may be employed in obtaining the polymerization prodcient to accomplish the desired degree of polymerization. Agitation of the reaction mixture helps to increase the rate of reaction. Catalysts, mainly of an oxidizing type, such as hydrogen peroxide, sodium perborate, potassium persulfate,

or the like serve to promote the rate of reaction.`

Other agents may be added to modify or favorably influence the polymerization process, such as xanthogen disulfides, higher alkyl mercaptans (benzyl mercaptan oi`octyl mercaptan), or the like. Optimum temperatures for the reaction generally employed lie within the range of about 30 C. to 60 C. As a general rule, the water employed in the emulsion is in the major proportion of the reactants, and one reactant may be in various proportions with respect to any coreactant. Generally, the conversion requires a reaction time of several hours, from approximately 6 to 15 hours.

A stabilizing agent is added to the polymerization product, generally and preferably before the coagulation step. As effective stabilizers may be mentioned diphenylamine, phenyl-beta-naphthylamine, and condensation-products of diarylamines with ketones.

The following example illustrate the invention without limiting it thereto: l

A synthetic latex-like emulsion was prepared by reacting 30 parts by weight of conjugated butadien with 1o parts by weight, of acrylonlhile emulsified in parts by weight of water containing 1.8% of sodium stearate as the emulsiiier and 0.25% of chemically pure benzyl mercaptan. The reaction was carried out at about 40 C. forabout 'I3/4 hours, and the yield obtained was about '15% of the theoretical yield. -Therewas then added Y 0.6 part by weight of phenyl-beta-naphthylaminedispersed in a 1.8% solution of sodium stearate. The polymerization product in the emulsion thus prepared was coagulated by atomizing the emulsion into a bath of liquid isobutylene at 6 C. thru a nozzle orifice less than 0.0001 inch in dlameter. of very fine particles of coagulate was formed. The coagulate was readily -washed free of emulsier by one reslurry operation with distilled water, and was .thereafter 'easily filtered and readily dried in a current of heated air.

In the same manner but with liquefied butal diene at about 4 C., replacing the liqueiied isobutylene as the coagulant, an equally satisfactory coagulation of the described product was accomplished.

The present method of coagulation 'was found excellently adaptable also for bringing about decomposition of a soap emulsiiier when it is desired to have a small amount of free fatty acid liberated .from `the soap to remain with the polymer.

For example, in another run, the synthetic latex-like emulsion was sprayed into liquid isobutylene at about 6 C. containing a small amount of glacial acetic acid, and it was demonstrated that the fatty acid soap emulslfying agent, sodium stearate, was made to liberate free limited amount of the stearic acid, e. g., less than 5 parts of the acid per 100 parts of coagulate, and preferably about 2 parts to 100 parts of coagulate.

In the foregoing examples, the products obtained after washing and drying the coagulate and subsequent compounding and curing with sulfur were found fully'equal in tensile strength and elasticity to products 'obtained by the usual' `strength after identical compounding and curing treatments. The brine precipitated ,coagulate product when compounded and cured showed a tensile strength of 2850 lbs/sq. in., whereas the coagulate formed by spraying theA emulsion into liquid isobutylene at about -6 C. yielded a product having a tensile strengthof 3680 lbs/sq. in.

The coagulated rubber-like masses thus ob- 4 tainable have all the desired qualities of plasticity when worked on roll mills when being .admixed In the liquid isobutylene, a. suspension Tins-coagulation technique is readily applicablel with equipment at present available, and advantageously dispenses with the need of expensive special equipment for washing and drying since it very definitely can be used to form a coagulate in a form which is excellently adapted for a simple washing and drying. The improvedcoagulate products of controlled particle size and proper state of purity are highly desirable in compounding operations.

Although specic examples and preferred embodiments illustrating the invention have been given, it is to be understood that other modifi- .cations come withinthe spirit and scope of this zone, and contacting a dispersed stream of said latex with said refrigerant in said zone whereby said latex is frozen, and thawing said frozen latex to separate synthetic rubber therefrom.

2. The process of coagulating synthetic rubber latices obtained by the copolymerization of a conjugated diolefln and an unsaturated comonomer in aqueous emulsion which comprises subdividing said latex into small drops, contacting said small drops with a non-polar organic coagulant liquid which is immiscible with aqueous media and inert to the polymer under the coagulating conditions and which is maintained by evaporation at a sufficiently low temperature to freeze water and thereby induce coagulation of the polymer particles.

3. The method as defined in claim 2 in which the coagulant liquid is liquefied isobutylene.

4. The method as defined in claim 2 in which the coagulant liquid is liquefied butadiene.

5. The process of coagulating synthetic rubber latices obtained by the copolymerization of a conjugated dioleiln and an unsaturated comonomer in aqueous emulsion which comprises atomizing said latices in to small drops, distributing said drops into a bath of a volatile, non-polar organic coagulant liquid which is immiscible with aqueous media and inert to the polymer under the coagulating conditionsl and which is maintained by evaporation at a temperature sufficiently low to freeze the latex and form a slurry of ice and coagulated latex particles suspended in' said orvganic coagulant liquid, passing said slurry into contact with water at -a temperature sufllciently high to melt the ice particles and vaporize said `organic liquid while the coagulated polymer forms a second slurry by suspension in the warm water, and stripping the volatile coagulant liquid from said second slurry.

6. The process as defined in claim 5 wherein the copolymer is separated from the second water slurry in washed condition and then dried.

7. The process as dened in claim 5 wherein the copolymer is prepared from a mixture of butadiene and acrylonitrile.

8. The 'process as defined in claim 5 wherein the copolymer is prepared from a mixture of butadiene and styrene.

9. The process of coagulating synthetic rubber latices obtained by the copolymerization of a conjugated diolen and an unsaturated comonomer in aqueous emulsion using a fatty acid soap emulsifier which comprises atomizing said latices into small drops, distributing said drops into a bath of a volatile, non-polar, organic coagulant. liquid which is immiscible with aqueous media and inert to the polymer under the coagulating conditions and which contains a small amount of an agent which liberates fatty acid from said soap, and which coagulant liquid is maintained by evaporation at a temperature sumciently low to freeze the latex and thereby induce coagulation of the polymer particles and recovering the coagulated polymer particles containing from 2 to 5 parts of said fatty-acid per 100 parts of coagulate.y

ROBERT M. THOMAS. DONALD C. FIELD.

REFERENCES 'CITED The following references are of record in the le'of this patent:

UNITED STATES PATENTS Number Name Date 2,234,204 Starkweather et a1. Mar. 11, 1941 2,187,146 Calcott et al Jan. 16, 1940 2,085,524 DeSimo June 29, 1937 l 

